Dispersion is a phenomenon whereby different optical wavelengths travel at different speeds through a dispersive media such as glass. And since a modulated carrier signal comprises many wavelengths, the optical signal that emerges from the distant end of a glass fiber is a smeared version of the signal that was launched into the near end. In the case of linear dispersion, this is solved by periodically providing compensation along an optical fiber route, and fewer compensation stages are better.
Conventional singlemode fiber systems primarily operate in the wavelength region between 1285 and 1335 nanometers (nm) and have a zero-dispersion wavelength at about 1310 nm. However, the optical fiber used in such systems is poorly suited for transmitting multiple closely spaced carrier wavelengths because of nonlinear interactions and mixing between the channels. The limiting form of such nonlinear phenomena--4-photon mixing (4PM)--is described in the literature (see e.g., article by D. Marcuse, A. Chraplyvy and R. Tkach entitled: "Effect of Fiber Nonlinearity on Long-Distance Transmission," Journal of Lightwave Technology, vol. 9, No. 1, January 1991, pp. 121-128). Briefly, 4PM appears as a fluctuating gain or loss due to constructive and destructive interference between different signal channels. The magnitude of 4PM is power dependent and may be reduced by decreasing launch power.
Multi-channel optical systems provide the most efficient use of an optical fiber and include wavelength-division multiplexers, which operate to combine an number of closely spaced channels (wavelength regions) onto a single optical path in one direction of transmission, and to separate them from the optical path in the other direction of transmission. And while conventional singlemode fiber systems do provide WDM operation in the 1.55 .mu.m wavelength region, there is too much linear dispersion (e.g., about 17 ps/nm-km) that needs to be compensated. For example, compensation is required every 50 to 100 kilometers, which is an impractical short distance.
Contemplated uses of optical fiber include the transmission of all type of digital and analog information, both separately and together. Particular uses include data (such as Internet traffic) as well as broadcast television (TV) signals, which typically utilize amplitude modulated, vestigial-sideband (AM-VSB) modulation. Analog signals are inherently noise sensitive, and noise is readily observable in TV pictures. In particular, when multiple wavelengths such as WDM signals are transmitted on a single fiber, stimulated Raman scattering (SRS) causes energy to be transferred from the WDM signals into another wavelength region that is as much as 120 nm longer. At the present time, there are no systems that provide WDM and analog TV signals over the same optical fiber.
It has been observed in the article entitled: Fabrication Of Completely OH-Free V.A.D. Fiber in Electronics Letters, Aug. 28, 1980 Vol. 16 No. 19, that a completely OH-free optical fiber, with no loss peaks due to OH ions at any wavelengths in the loss spectrum from ultraviolet to infrared, has been desired for some time; and that such a fiber will play an important role as a transmission medium for WDM systems. However, this article provides no information regarding the dispersion characteristics of such a fiber, and it provides no information regarding the allocation of optical channels within the usable loss spectrum of optical fiber.
For these and other reasons, an optical transmission system that is compatible with apparatus that was designed for conventional singlemode fiber systems, which permits WDM operation without 4PM interference among WDM signals, and which avoids SRS interference between WDM and analog TV signals, would be of great interest. This application discloses such a system.